Caveat: The following haven't been tested by me!
Try connecting a monitor/TV to the HDMI output and:
See if the
xrandr
utility displays the relevant information.Search the /var/log/Xorg.0.log file for all instances of the string "HDMI" (case insensitive of course), then check all the lines in the vicinity. (You might need to copy the file using sudo, change the extension to .txt and also edit the permissions so you can read it.)